1. Field of the Invention
The present invention relates to an electrode for a fuel cell, comprising a proton conductor, a membrane electrode composite, and a fuel cell comprising a membrane electrode composite. The present invention also relates to a method for manufacturing them.
2. Background Art
Fuel cells comprise a fuel electrode (anode) provided on one side of a proton-conductive film and an oxidizing agent (oxidizer) electrode (cathode) provided on the other side of the proton-conductive film. Upon the supply of a fuel such as hydrogen or methanol to the anode, the fuel is electrochemically oxidized in the anode to generate protons and electrons. The generated electrons flow into an external circuit. The protons thus produced arrive at the cathode through the proton-conductive film and are reacted with an oxidizing agent supplied to the cathode and electrons from an external circuit to produce water.
Excellent proton-conductive properties are required of both the anode and cathode. For example, perfluorosulfonic acid-containing polymers (for example, Nafion (tradename, manufactured by Du Pont)) are known as the proton conductor. From the viewpoint of reducing fuel cell systems, for example, methanol as a liquid fuel is in many cases used in a high concentration. The perfluorosulfonic acid-containing polymer, when used as the proton-conductive binder in an electrode catalyst layer, is dissolved in highly concentrated methanol. In particular, the dissolution of the perfluorosulfonic acid-containing polymer is promoted by power generation at a high temperature of 100° C. or above, which provides high output, or by a temperature rise caused by heat generation involved in the power generation. Accordingly, it is difficult to provide stable output.
A sulfuric acid-supported metal oxide having solid superacidity is known as an inorganic solid acid-based proton conductor (see, for example, JP-A 2004-158261 (KOKAI)). Specifically, the sulfuric acid-supported metal oxide has been produced by supporting sulfuric acid on the surface of an oxide containing at least one element selected from zirconium, titanium, iron, tin, silicon, aluminum, molybdenum, and tungsten and heat treating the assembly to fix sulfuric acid on the surface of the oxide. The sulfuric acid-supported metal oxide develops proton conductivity by virtue of the fixed sulfate group. However, the sulfate group is eliminated by hydrolysis, and the proton conductivity is lowered. Accordingly, the proton-conductive solid electrolyte is unstable as a proton-conductive solid electrolyte in a fuel cell which produces water in the course of power generation, particularly a fuel cell using a liquid fuel, and thus is possibly unsuitable for stable supply of electric power for a long period of time.
Further, Eiji Higuchi, Hiroyuki Uchida, Tatsuo Fujinami, and Masahiro Watanabe et al., Solid State Ionics, 171, 45-49 (2004) describes a sulfonic acid group-containing borosiloxane proton-conductive solid electrolyte as a proton-conductive inorganic binder in the electrode catalyst layer. In this case, a sol solution of a metal alkoxide as a starting material of the sulfonic acid group-containing borosiloxane is mixed with catalyst particles, and the resultant slurry is coated onto carbon paper, and the coating is heat treated for use of the sulfonic acid group-containing borosiloxane proton-conductive solid electrolyte as a binder for the catalyst layer. As with the perfluorosulfonic acid-containing polymer, the sulfonic acid group-containing borosiloxane electrolyte requires a large amount of water (carrier water) for proton conduction through the sulfonic acid group. In the power generation under high temperatures at which it is difficult to ensure water, water necessary for proton conduction is reduced, and the proton conductivity is significantly lowered. Further, since there is a possibility that the sulfonic acid group is eliminated, the sulfonic acid group-containing borosiloxane electrolyte is considered as a material unsuitable for stable supply of electric power for a long period of time.
Furthermore, JP-A 2001-102071 (KOKAI) describes that an inorganic glass containing P2O5 and SiO2 is used as a proton-conductive inorganic material in an electrode catalyst layer. In this case, a sol or wet gel containing metal alcolates as starting materials for the inorganic glass containing P2O5 and SiO2 is coated onto the surface of each electrode in a fuel electrode and an oxidizer electrode, and the coating is dried and heated to bind the catalyst layer to prepare electrodes. The inorganic glass containing P2O5 and SiO2 utilizes OH groups on the glass surface for proton conduction. In the operation under high temperatures, however, the OH groups are eliminated upon drying resulting in lowered proton conductivity. Further, since there is a possibility that the P2O5 component for glass skeleton formation is dissolved in water, it is difficult to stably supply electric power for a long period of time.
The possession of high electron conductivity and proton conductivity by satisfactorily forming the continuity between catalysts or catalyst supporting materials, or the continuity of the proton conductor is necessary in the anode and cathode. Further, a number of three-phase interfaces as a reaction field should be formed by bringing the electron conductor into contact with the proton conductor. JP-A 2005-149742 (KOKAI) describes that an electroconductive metal oxide is used in a catalyst carrier in an electrode catalyst layer. In this case, the effect of long-term stability is recognized. Since, however, the catalyst carrier does not have any proton conductivity, it is considered that the utilization efficiency of the catalyst and the formation of the three-phase interfaces are unsatisfactory.
The present inventors have proposed, for example, electrodes for fuel cells, comprising an oxide carrier such as titanium (Ti), particles of an oxide of tungsten (W) or the like, a catalyst-supported carbon, and a polymer binder for binding them to each other (JP-A 2006-032287 (KOKAI)). In such electrodes for fuel cells and the like, water can easily be controlled, and it is considered that stable ion conductivity can be maintained over a temperature range from room temperature to a high temperature around 150° C.
An electrolyte membrane prepared by compositing an oxide of titanium (Ti), tungsten (W) or the like (an oxide superacid), a catalyst-supported carbon, and an organic polymer binder with each other is considered to have a structure in which the continuity between catalyst-supported carbons, the continuity between oxide superacids, and the continuity between the catalyst-supported carbon and the oxide superacid are less likely to be formed. Consequently, there is a possibility that the three-phase interface which causes an electrode reaction cannot be satisfactorily formed.